PostgreSQL 9.4 introduced WITH ORDINALITY for generating a sequence number for each element which are produced by UNNEST().

When we are talking about the string array, internal element numbering is also important for further operation.

I have prepared a small demonstration of this, you can access here.

Create a table with sample data:

CREATE TABLE tbl_TextArray(key varchar(100), tags text[]);

INSERT INTO tbl_TextArray(key, tags)
VALUES 
('first', '{aa,fe,gr,fh}'::text[]),
('sec', '{tr,we,et,ui,er}'::text[]),
('third', '{we,gh,fv,dc,sx}'::text[]);

UNNEST() WITH ORDINALITY:

SELECT key, ArrayOrder, tag
FROM tbl_TextArray, unnest(tags) 
WITH ORDINALITY As T (tag, ArrayOrder);

The Result:

dbrnd=# SELECT key, ArrayOrder, tag
dbrnd-# FROM tbl_TextArray, unnest(tags)
dbrnd-# WITH ORDINALITY As T (tag, ArrayOrder);
  key  | arrayorder | tag
-------+------------+-----
 first |          1 | aa
 first |          2 | fe
 first |          3 | gr
 first |          4 | fh
 sec   |          1 | tr
 sec   |          2 | we
 sec   |          3 | et
 sec   |          4 | ui
 sec   |          5 | er
 third |          1 | we
 third |          2 | gh
 third |          3 | fv
 third |          4 | dc
 third |          5 | sx
(14 rows)